Philippe Golle and Kurt Partridge of PARC have a cute paper (pdf) on the anonymity of geo-location data. They analyze data from the U.S. Census and show that for the average person, knowing their approximate home and work locations — to a block level — identifies them uniquely.

Even if we look at the much coarser granularity of a census tract — tracts correspond roughly to ZIP codes; there are on average 1,500 people per census tract — for the average person, there are only around 20 other people who share the same home and work location. There’s more: 5% of people are uniquely identified by their home and work locations even if it is known only at the census tract level. One reason for this is that people who live and work in very different areas (say, different counties) are much more easily identifiable, as one might expect.

The paper is timely, because Location Based Services  are proliferating rapidly. To understand the privacy threats, we need to ask the two usual questions:

1. who has access to anonymized location data?
2. how can they get access to auxiliary data linking people to location pairs, which they can then use to carry out re-identification?

The authors don’t say much about these questions, but that’s probably because there are too many possibilities to list! In this post I will examine a few.

GPS navigation. This is the most obvious application that comes to mind, and probably the most privacy-sensitive: there have been many controversies around tracking of vehicle movements, such as NYC cab drivers threatening to strike. The privacy goal is to keep the location trail of the user/vehicle unknown even to the service provider — unlike in the context of social networks, people often don’t even trust the service provider. There are several papers on anonymizing GPS-related queries, but there doesn’t seem to be much you can do to hide the origin and destination except via charmingly unrealistic cryptographic protocols.

The accuracy of GPS is a few tens or few hundreds of feet, which is the same order of magnitude as a city block. So your daily commute is pretty much unique. If you took a (GPS-enabled) cab home from work at a certain time, there’s a good chance the trip can be tied to you. If you made a detour to stop somewhere, the location of your stop can probably be determined. This is true even if there is no record tying you to a specific vehicle.

Location based social networking. Pretty soon, every smartphone will be capable of running applications that transmit location data to web services. Google Latitude and Loopt are two of the major players in this space, providing some very nifty social networking functionality on top of location awareness. It is quite tempting for service providers to outsource research/data-mining by sharing de-identified data. I don’t know if anything of the sort is being done yet, but I think it is clear that de-identification would offer very little privacy protection in this context. If a pair of locations is uniquely identifying, a trail is emphatically so.

The same threat also applies to data being subpoena’d, so data retention policies need to take into consideration the uselessness of anonymizing location data.

I don’t know if cellular carriers themselves collect a location trail from phones as a matter of course. Any idea?

Plain old web browsing. Every website worth the name identifies you with a cookie, whether you log in or not. So if you browse the web from a laptop or mobile phone from both home and work, your home and work IP addresses can be tied together based on the cookie. There are a number of free or paid databases for turning IP addresses into geographical locations. These are generally accurate up to the city level, but beyond that the accuracy is shaky.

A more accurate location fix can be obtained by IDing WiFi access points. This is a curious technological marvel that is not widely known. Skyhook, Inc. has spent years wardriving the country (and abroad) to map out the MAC addresses of wireless routers. Given the MAC address of an access point, their database can tell you where it is located. There are browser add-ons that query Skyhook’s database and determine the user’s current location. Note that you don’t have to be browsing wirelessly — all you need is at least one WiFi access point within range. This information can then be transmitted to websites which can provide location-based functionality; Opera, in particular, has teamed up with Skyhook and is “looking forward to a future where geolocation data is as assumed part of the browsing experience.” The protocol by which the browser communicates geolocation to the website is being standardized by the W3C.

The good news from the privacy standpoint is that the accurate geolocation technologies like the Skyhook plug-in (and a competing offering that is part of Google Gears) require user consent. However, I anticipate that once the plug-ins become common, websites will entice users to enable access by (correctly) pointing out that their location can only be determined to within a few hundred meters, and users will leave themselves vulnerable to inference attacks that make use of location pairs rather than individual locations.

Image metadata. An increasing number of cameras these days have (GPS-based) geotagging built-in and enabled by default. Even more awesome is the Eye-Fi card, which automatically uploads pictures you snap to Flickr (or any of dozens of other image sharing websites you can pick from) by connecting to available WiFi access points nearby. Some versions of the card do automatic geotagging in addition.

If you regularly post pseudonymously to (say) Flickr, then the geolocations of your pictures will probably reveal prominent clusters around the places you frequent, including your home and work. This can be combined with auxiliary data to tie the pictures to your identity.

Now let us turn to the other major question: what are the sources of auxiliary data that might link location pairs to identities? The easiest approach is probably to buy data from Acxiom, or another provider of direct-marketing address lists. Knowing approximate home and work locations, all that the attacker needs to do is to obtain data corresponding to both neighborhoods and do a “join,” i.e, find the (hopefully) unique common individual. This should be easy with Axciom, which lets you filter the list by  “DMA code, census tract, state, MSA code, congressional district, census block group, county, ZIP code, ZIP range, radius, multi-location radius, carrier route, CBSA (whatever that is), area code, and phone prefix.”

Google and Facebook also know my home and work addresses, because I gave them that information. I expect that other major social networking sites also have such information on tens of millions of users. When one of these sites is the adversary — such as when you’re trying to browse anonymously — the adversary already has access to the auxiliary data. Google’s power in this context is amplified by the fact that they own DoubleClick, which lets them tie together your browsing activity on any number of different websites that are tracked by DoubleClick cookies.

Finally, while I’ve talked about image data being the target of de-anonymization, it may equally well be used as the auxiliary information that links a location pair to an identity — a non-anonymous Flickr account with sufficiently many geotagged photos probably reveals an identifiable user’s home and work locations. (Some attack techniques that I describe on this blog, such as crawling image metadata from Flickr to reveal people’s home and work locations, are computationally expensive to carry out on a large scale but not algorithmically hard; such attacks, as can be expected, will rapidly become more feasible with time.)

Summary. A number of devices in our daily lives transmit our physical location to service providers whom we don’t necessarily trust, and who keep might keep this data around or transmit it to third parties we don’t know about. The average user simply doesn’t have the patience to analyze and understand the privacy implications, making anonymity a misleadingly simple way to assuage their concerns. Unfortunately, anonymity breaks down very quickly when more than one location is associated with a person, as is usually the case.